Accessing a migrated member in an updated type

ABSTRACT

Accessing migrated members in an updated type is described. Instructions to access a migrated member may be: (a) storing a value of a particular type as a value of a migrated field, or (b) invoking a migrated method using an argument of a particular type. The argument of the particular type, specified in the instructions, is converted into a value of the type associated with the current version of the migrated member. The migrated member is accessed using the converted value. Alternatively, instructions may be: (a) fetching and returning a value of a migrated field as a value of a particular type, or (b) returning a value from a migrated method as a value of a particular type. A value is returned via accessing the current version of the migrated member. The returned value is converted into a value of the particular type specified in the instructions.

BENEFIT CLAIM; INCORPORATION BY REFERENCE

This application claims the benefit of U.S. Provisional Patent Application No. 62/361,087, filed Jul. 12, 2016, which is hereby incorporated by reference.

This application is related to U.S. Non-Provisional patent application Ser. No. 15/426,839, filed Feb. 7, 2017, now U.S. Pat. No. 10,635,420, which is hereby incorporated by reference.

TECHNICAL FIELD

The present disclosure relates to compiling and/or executing a type. In particular, the present disclosure relates to accessing a migrated member of an updated type.

BACKGROUND

A compiler converts source code, which is written according to a specification directed to the convenience of the programmer, to either machine or object code, which is executable directly by the particular machine environment, or an intermediate representation (“virtual machine code/instructions”), such as bytecode, which is executable by a virtual machine that is capable of running on top of a variety of particular machine environments. The virtual machine instructions are executable by the virtual machine in a more direct and efficient manner than the source code. Converting source code to virtual machine instructions includes mapping source code functionality from the language to virtual machine functionality that utilizes underlying resources, such as data structures. Often, functionality that is presented in simple terms via source code by the programmer is converted into more complex steps that map more directly to the instruction set supported by the underlying hardware on which the virtual machine resides.

The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments are illustrated by way of example and not by way of limitation in the figures of the accompanying drawings. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and they mean at least one. In the drawings:

FIG. 1 illustrates an example computing architecture in which techniques described herein may be practiced.

FIG. 2 is a block diagram illustrating one embodiment of a computer system suitable for implementing methods and features described herein.

FIG. 3 illustrates an example virtual machine memory layout in block diagram form according to an embodiment.

FIG. 4 illustrates an example frame in block diagram form according to an embodiment.

FIG. 5 illustrates an example of an updated type, in accordance with one or more embodiments;

FIG. 6 illustrates a set of operations for compiling an updated type comprising an migrated member, in accordance with one or more embodiments.

FIGS. 7A-B illustrate a set of operations for fetching and returning a value of a migrated field as a value of a particular type, in accordance with one or more embodiments.

FIGS. 8A-B illustrate a set of operations for storing a value of a particular type as a value of a migrated field, in accordance with one or more embodiments.

FIGS. 9A-B illustrate a set of operations for invoking a method using values of a particular set of types as arguments to the method, in accordance with one or more embodiments.

FIGS. 10A-B illustrate a set of operations for returning a value from a method as a value of a particular type, in accordance with one or more embodiments.

FIG. 11 illustrates a system in accordance with one or more embodiments.

DETAILED DESCRIPTION

In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding. One or more embodiments may be practiced without these specific details. Features described in one embodiment may be combined with features described in a different embodiment. In some examples, well-known structures and devices are described with reference to a block diagram form in order to avoid unnecessarily obscuring the present invention.

-   -   1. GENERAL OVERVIEW     -   2. ARCHITECTURAL OVERVIEW         -   2.1 EXAMPLE CLASS FILE STRUCTURE         -   2.2 EXAMPLE VIRTUAL MACHINE ARCHITECTURE         -   2.3 LOADING, LINKING, AND INITIALIZING     -   3. AN UPDATED TYPE     -   4. COMPILING AN UPDATED TYPE     -   5. ACCESSING A MIGRATED FIELD     -   6. ACCESSING A MIGRATED METHOD     -   7. EXAMPLE EMBODIMENTS     -   8. MISCELLANEOUS; EXTENSIONS     -   9. HARDWARE OVERVIEW

1. GENERAL OVERVIEW

One or more embodiments include accessing a migrated member in an updated type. A migrated member (such as a migrated field or a migrated method) is a member for which multiple versions exist. Multiple versions of the member within the same updated type may include, for example, (a) a current version and (b) an outdated version which has been migrated to the current version. Each version of the migrated member is associated with a same member name. An updated type, as referred to herein for purposes of explanation, is a type that includes multiple versions of a migrated member.

In an embodiment, accessing a migrated field includes fetching and returning a value of a migrated field, corresponding to an object, as a value of a particular type. The field type associated with a current version of the migrated field is determined to be different from the particular type. Responsive to determining that the field type associated with the current version of the migrated field is different from the particular type, the value of the migrated field is converted into a value of the particular type. The converted value is stored as the value of the particular type.

In an embodiment, accessing a migrated field includes storing a value of a particular type as a value of a migrated field corresponding to an object. The field type associated with a current version of the migrated field is determined to be different from the particular type. Responsive to determining that the field type associated with the current version of the migrated field is different from the particular type, the value of the particular type is converted into a value of the field type associated with the current version of the migrated field. The converted value is stored as the value of the current version of the migrated field.

In an embodiment, accessing a migrated method includes invoking a migrated method using values of a particular set of types as arguments for the method. A set of parameter types associated with a current version of the migrated method is determined to be different from the particular set of types. Responsive to determining that the set of parameter types associated with the current version of the migrated method is different from the particular set of types, the values of the particular set of types are converted into values of the set of parameter types. The converted values are used as arguments for invoking the current version of the migrated method.

In an embodiment, accessing a migrated method includes returning a value from a migrated method as a value of a particular type. A return type associated with a current version of the migrated method is determined to be different from the particular type. Responsive to determining that the return type associated with the current version of the migrated method is different from the particular type, the value returned by the current version of the migrated method is converted into a value of the particular type. The converted value is stored as the value of the particular type.

One or more embodiments described in this Specification and/or recited in the claims may not be included in this General Overview section.

2. ARCHITECTURAL OVERVIEW

FIG. 1 illustrates an example architecture in which techniques described herein may be practiced. Software and/or hardware components described with relation to the example architecture may be omitted or associated with a different set of functionality than described herein. Software and/or hardware components, not described herein, may be used within an environment in accordance with one or more embodiments. Accordingly, the example environment should not be constructed as limiting the scope of any of the claims.

As illustrated in FIG. 1, a computing architecture 100 includes source code files 101 which are compiled by a compiler 102 into class files 103 representing the program to be executed. The class files 103 are then loaded and executed by an execution platform 112, which includes a runtime environment 113, an operating system 111, and one or more application programming interfaces (APIs) 110 that enable communication between the runtime environment 113 and the operating system 111. The runtime environment 113 includes a virtual machine 104 comprising various components, such as a memory manager 105 (which may include a garbage collector), a class file verifier 106 to check the validity of class files 103, a class loader 107 to locate and build in-memory representations of classes, an interpreter 108 for executing the virtual machine 104 code, and a just-in-time (JIT) compiler 109 for producing optimized machine-level code.

In an embodiment, the computing architecture 100 includes source code files 101 that contain code that has been written in a particular programming language, such as Java, C, C++, C#, Ruby, Perl, and so forth. Thus, the source code files 101 adhere to a particular set of syntactic and/or semantic rules for the associated language. For example, code written in Java adheres to the Java Language Specification. However, since specifications are updated and revised over time, the source code files 101 may be associated with a version number indicating the revision of the specification to which the source code files 101 adhere. The exact programming language used to write the source code files 101 is generally not critical.

In various embodiments, the compiler 102 converts the source code, which is written according to a specification directed to the convenience of the programmer, to either machine or object code, which is executable directly by the particular machine environment, or an intermediate representation (“virtual machine code/instructions”), such as bytecode, which is executable by a virtual machine 104 that is capable of running on top of a variety of particular machine environments. The virtual machine instructions are executable by the virtual machine 104 in a more direct and efficient manner than the source code. Converting source code to virtual machine instructions includes mapping source code functionality from the language to virtual machine functionality that utilizes underlying resources, such as data structures. Often, functionality that is presented in simple terms via source code by the programmer is converted into more complex steps that map more directly to the instruction set supported by the underlying hardware on which the virtual machine 104 resides.

In general, programs are executed either as a compiled or an interpreted program. When a program is compiled, the code is transformed globally from a first language to a second language before execution. Since the work of transforming the code is performed ahead of time; compiled code tends to have excellent run-time performance. In addition, since the transformation occurs globally before execution, the code can be analyzed and optimized using techniques such as constant folding, dead code elimination, inlining, and so forth. However, depending on the program being executed, the startup time can be significant. In addition, inserting new code would require the program to be taken offline, re-compiled, and re-executed. For many dynamic languages (such as Java) which are designed to allow code to be inserted during the program's execution, a purely compiled approach may be inappropriate. When a program is interpreted, the code of the program is read line-by-line and converted to machine-level instructions while the program is executing. As a result, the program has a short startup time (can begin executing almost immediately), but the run-time performance is diminished by performing the transformation on the fly. Furthermore, since each instruction is analyzed individually, many optimizations that rely on a more global analysis of the program cannot be performed.

In some embodiments, the virtual machine 104 includes an interpreter 108 and a JIT compiler 109 (or a component implementing aspects of both), and executes programs using a combination of interpreted and compiled techniques. For example, the virtual machine 104 may initially begin by interpreting the virtual machine instructions representing the program via the interpreter 108 while tracking statistics related to program behavior, such as how often different sections or blocks of code are executed by the virtual machine 104. Once a block of code surpasses a threshold (is “hot”), the virtual machine 104 invokes the JIT compiler 109 to perform an analysis of the block and generate optimized machine-level instructions which replaces the “hot” block of code for future executions. Since programs tend to spend most time executing a small portion of overall code, compiling just the “hot” portions of the program can provide similar performance to fully compiled code, but without the start-up penalty. Furthermore, although the optimization analysis is constrained to the “hot” block being replaced, there still exists far greater optimization potential than converting each instruction individually. There are a number of variations on the above described example, such as tiered compiling.

In order to provide clear examples, the source code files 101 have been illustrated as the “top level” representation of the program to be executed by the execution platform 112. Although the computing architecture 100 depicts the source code files 101 as a “top level” program representation, in other embodiments the source code files 101 may be an intermediate representation received via a “higher level” compiler that processed code files in a different language into the language of the source code files 101. Some examples in the following disclosure assume that the source code files 101 adhere to a class-based object-oriented programming language. However, this is not a requirement to utilizing the features described herein.

In an embodiment, compiler 102 receives as input the source code files 101 and converts the source code files 101 into class files 103 that are in a format expected by the virtual machine 104. For example, in the context of the JVM, the Java Virtual Machine Specification defines a particular class file format to which the class files 103 are expected to adhere. In some embodiments, the class files 103 contain the virtual machine instructions that have been converted from the source code files 101. However, in other embodiments, the class files 103 may contain other structures as well, such as tables identifying constant values and/or metadata related to various structures (classes, fields, methods, and so forth).

The following discussion assumes that each of the class files 103 represents a respective “type” defined in the source code files 101 (or dynamically generated by the compiler 102/virtual machine 104). Examples of “types” include a class and an interface. A class is a template for the properties and behaviors of objects associated with the class. The class includes fields and methods associated with the objects of the class. An interface includes abstract methods that may be implemented by a class. A class that implements an interface inherits the abstract methods of the interface and provides a body to each abstract method. However, the aforementioned assumption is not a strict requirement and will depend on the implementation of the virtual machine 104. Thus, the techniques described herein may still be performed regardless of the exact format of the class files 103.

Types may be updated to include multiple fields with a same field name but different field types. Additionally or alternatively, types may be updated to include multiple methods with a same method name but different parameter types and/or return types. Embodiments herein relate to accessing a member of an updated type.

In some embodiments, the class files 103 are divided into one or more “libraries” or “packages”, each of which includes a collection of classes that provide related functionality. For example, a library may contain one or more class files that implement input/output (I/O) operations, mathematics tools, cryptographic techniques, graphics utilities, and so forth. Further, some classes (or fields/methods within those classes) may include access restrictions that limit their use to within a particular class/library/package or to classes with appropriate permissions.

2.1 Example Class File Structure

FIG. 2 illustrates an example structure for a class file 200 in block diagram form according to an embodiment. In order to provide clear examples, the remainder of the disclosure assumes that the class files 103 of the computing architecture 100 adhere to the structure of the example class file 200 described in this section. However, in a practical environment, the structure of the class file 200 will be dependent on the implementation of the virtual machine 104. Further, one or more features discussed herein may modify the structure of the class file 200 to, for example, add additional structure types. Therefore, the exact structure of the class file 200 is not critical to the techniques described herein. For the purposes of Section 2.1, “the class” or “the present class” refers to the class represented by the class file 200.

In FIG. 2, the class file 200 includes a constant table 201, field structures 208, class metadata 207, and method structures 209. In an embodiment, the constant table 201 is a data structure which, among other functions, acts as a symbol table for the class. For example, the constant table 201 may store data related to the various identifiers used in the source code files 101 such as type, scope, contents, and/or location. The constant table 201 has entries for value structures 202 (representing constant values of type int, long, double, float, byte, string, and so forth), class information structures 203, name and type information structures 204, field reference structures 205, and method reference structures 206 derived from the source code files 101 by the compiler 102. In an embodiment, the constant table 201 is implemented as an array that maps an index i to structure j. However, the exact implementation of the constant table 201 is not critical.

In some embodiments, the entries of the constant table 201 include structures which index other constant table 201 entries. For example, an entry for one of the value structures 202 representing a string may hold a tag identifying its “type” as string and an index to one or more other value structures 202 of the constant table 201 storing char, byte or int values representing the ASCII characters of the string.

In an embodiment, field reference structures 205 of the constant table 201 hold an index into the constant table 201 to one of the class information structures 203 representing the class defining the field and an index into the constant table 201 to one of the name and type information structures 204 that provides the name and descriptor of the field. Method reference structures 206 of the constant table 201 hold an index into the constant table 201 to one of the class information structures 203 representing the class defining the method and an index into the constant table 201 to one of the name and type information structures 204 that provides the name and descriptor for the method. The class information structures 203 hold an index into the constant table 201 to one of the value structures 202 holding the name of the associated class.

The name and type information structures 204 hold an index into the constant table 201 to one of the value structures 202 storing the name of the field/method and an index into the constant table 201 to one of the value structures 202 storing the descriptor.

In an embodiment, class metadata 207 includes metadata for the class, such as version number(s), number of entries in the constant pool, number of fields, number of methods, access flags (whether the class is public, private, final, abstract, etc.), an index to one of the class information structures 203 of the constant table 201 that identifies the present class, an index to one of the class information structures 203 of the constant table 201 that identifies the superclass (if any), and so forth.

In an embodiment, the field structures 208 represent a set of structures that identifies the various fields of the class. The field structures 208 store, for each field of the class, accessor flags for the field (whether the field is static, public, private, final, etc.), an index into the constant table 201 to one of the value structures 202 that holds the name of the field, and an index into the constant table 201 to one of the value structures 202 that holds a descriptor of the field.

In an embodiment, the method structures 209 represent a set of structures that identifies the various methods of the class. The method structures 209 store, for each method of the class, accessor flags for the method (e.g. whether the method is static, public, private, synchronized, etc.), an index into the constant table 201 to one of the value structures 202 that holds the name of the method, an index into the constant table 201 to one of the value structures 202 that holds the descriptor of the method, and the virtual machine instructions that correspond to the body of the method as defined in the source code files 101.

In an embodiment, a descriptor represents a type of a field or method. For example, the descriptor may be implemented as a string adhering to a particular syntax. While the exact syntax is not critical, a few examples are described below.

In an example where the descriptor represents a type of the field, the descriptor identifies the type of data held by the field. In an embodiment, a field can hold a basic type, an object, or an array. When a field holds a basic type, the descriptor is a string that identifies the basic type (e.g., “B”=byte, “C”=char, “D”=double, “F”=float, “I”=int, “J”=long int, etc.). When a field holds an object, the descriptor is a string that identifies the class name of the object (e.g. “L ClassName”). “L” in this case indicates a reference, thus “L ClassName” represents a reference to an object of class ClassName. When the field is an array, the descriptor identifies the type held by the array. For example, “[B” indicates an array of bytes, with “[” indicating an array and “B” indicating that the array holds the basic type of byte. However, since arrays can be nested, the descriptor for an array may also indicate the nesting. For example, “[[L ClassName” indicates an array where each index holds an array that holds objects of class ClassName. In some embodiments, the ClassName is fully qualified and includes the simple name of the class, as well as the pathname of the class. For example, the ClassName may indicate where the file is stored in the package, library, or file system hosting the class file 200.

In the case of a method, the descriptor identifies the parameters of the method and the return type of the method. For example, a method descriptor may follow the general form “({ParameterDescriptor}) ReturnDescriptor”, where the {ParameterDescriptor} is a set or list of field descriptors representing the parameters and the ReturnDescriptor is a field descriptor identifying the return type. For instance, the string “V” may be used to represent the void return type. Thus, a method defined in the source code files 101 as “Object m(int I, double d, Thread t) { . . . }” matches the descriptor “(I D L Thread) L Object”.

In an embodiment, the virtual machine instructions held in the method structures 209 include operations which reference entries of the constant table 201. Using Java as an example, consider the following class:

class A

{

-   -   int add12and13( ) {         -   return B.addTwo(12, 13);         -   }

}

In the above example, the Java method add12and13 is defined in class A, takes no parameters, and returns an integer. The body of method add12and13 calls static method addTwo of class B which takes the constant integer values 12 and 13 as parameters, and returns the result. Thus, in the constant table 201, the compiler 102 includes, among other entries, a method reference structure that corresponds to the call to the method B.addTwo. In Java, a call to a method compiles down to an invoke command in the bytecode of the JVM (in this case invokestatic as addTwo is a static method of class B). The invoke command is provided an index into the constant table 201 corresponding to the method reference structure that identifies the class defining addTwo “B”, the name of addTwo “addTwo”, and the descriptor of addTwo “(I I)I”. For example, assuming the aforementioned method reference is stored at index 4, the bytecode instruction may appear as “invokestatic #4”.

Since the constant table 201 refers to classes, methods, and fields symbolically with structures carrying identifying information, rather than direct references to a memory location, the entries of the constant table 201 are referred to as “symbolic references”. One reason that symbolic references are utilized for the class files 103 is because, in some embodiments, the compiler 102 is unaware of how and where the classes will be stored once loaded into the runtime environment 113. As will be described in Section 2.3, eventually the run-time representations of the symbolic references are resolved into actual memory addresses by the virtual machine 104 after the referenced classes (and associated structures) have been loaded into the runtime environment and allocated concrete memory locations.

2.2 Example Virtual Machine Architecture

FIG. 3 illustrates an example virtual machine memory layout 300 in block diagram form according to an embodiment. In order to provide clear examples, the remaining discussion will assume that the virtual machine 104 adheres to the virtual machine memory layout 300 depicted in FIG. 3. In addition, although components of the virtual machine memory layout 300 may be referred to as memory “areas”, there is no requirement that the memory areas are contiguous.

In the example illustrated by FIG. 3, the virtual machine memory layout 300 is divided into a shared area 301 and a thread area 307. The shared area 301 represents an area in memory where structures shared among the various threads executing on the virtual machine 104 are stored. The shared area 301 includes a heap 302 and a per-class area 303. In an embodiment, the heap 302 represents the run-time data area from which memory for class instances and arrays is allocated. In an embodiment, the per-class area 303 represents the memory area where the data pertaining to the individual classes are stored. In an embodiment, the per-class area 303 includes, for each loaded class, a run-time constant pool 304 representing data from the constant table 201 of the class, field and method data 306 (for example, to hold the static fields of the class), and the method code 305 representing the virtual machine instructions for methods of the class.

The thread area 307 represents a memory area where structures specific to individual threads are stored. In FIG. 3, the thread area 307 includes thread structures 308 and thread structures 311, representing the per-thread structures utilized by different threads. In order to provide clear examples, the thread area 307 depicted in FIG. 3 assumes two threads are executing on the virtual machine 104. However, in a practical environment, the virtual machine 104 may execute any arbitrary number of threads, with the number of thread structures scaled accordingly.

In an embodiment, thread structures 308 includes program counter 309 and virtual machine stack 310. Similarly, thread structures 311 includes program counter 312 and virtual machine stack 313. In an embodiment, program counter 309 and program counter 312 store the current address of the virtual machine instruction being executed by their respective threads.

Thus, as a thread steps through the instructions, the program counters are updated to maintain an index to the current instruction. In an embodiment, virtual machine stack 310 and virtual machine stack 313 each store frames for their respective threads that hold local variables and partial results, and is also used for method invocation and return.

In an embodiment, a frame is a data structure used to store data and partial results, return values for methods, and perform dynamic linking. A new frame is created each time a method is invoked. A frame is destroyed when the method that caused the frame to be generated completes. Thus, when a thread performs a method invocation, the virtual machine 104 generates a new frame and pushes that frame onto the virtual machine stack associated with the thread.

When the method invocation completes, the virtual machine 104 passes back the result of the method invocation to the previous frame and pops the current frame off of the stack. In an embodiment, for a given thread, one frame is active at any point. This active frame is referred to as the current frame, the method that caused generation of the current frame is referred to as the current method, and the class to which the current method belongs is referred to as the current class.

FIG. 4 illustrates an example frame 400 in block diagram form according to an embodiment. In order to provide clear examples, the remaining discussion will assume that frames of virtual machine stack 310 and virtual machine stack 313 adhere to the structure of frame 400.

In an embodiment, frame 400 includes local variables 401, operand stack 402, and run-time constant pool reference table 403. In an embodiment, the local variables 401 are represented as an array of variables that each hold a value, for example, Boolean, byte, char, short, int, float, or reference. Further, some value types, such as longs or doubles, may be represented by more than one entry in the array. The local variables 401 are used to pass parameters on method invocations and store partial results. For example, when generating the frame 400 in response to invoking a method, the parameters may be stored in predefined positions within the local variables 401, such as indexes 1-N corresponding to the first to Nth parameters in the invocation.

In an embodiment, the operand stack 402 is empty by default when the frame 400 is created by the virtual machine 104. The virtual machine 104 then supplies instructions from the method code 305 of the current method to load constants or values from the local variables 401 onto the operand stack 402. Other instructions take operands from the operand stack 402, operate on them, and push the result back onto the operand stack 402. Furthermore, the operand stack 402 is used to prepare parameters to be passed to methods and to receive method results. For example, the parameters of the method being invoked could be pushed onto the operand stack 402 prior to issuing the invocation to the method. The virtual machine 104 then generates a new frame for the method invocation where the operands on the operand stack 402 of the previous frame are popped and loaded into the local variables 401 of the new frame. When the invoked method terminates, the new frame is popped from the virtual machine stack and the return value is pushed onto the operand stack 402 of the previous frame.

In an embodiment, the run-time constant pool reference table 403 contains a reference to the run-time constant pool 304 of the current class. The run-time constant pool reference table 403 is used to support resolution. Resolution is the process whereby symbolic references in the constant pool 304 are translated into concrete memory addresses, loading classes as necessary to resolve as-yet-undefined symbols and translating variable accesses into appropriate offsets into storage structures associated with the run-time location of these variables.

2.3 Loading, Linking, and Initializing

In an embodiment, the virtual machine 104 dynamically loads, links, and initializes classes. Loading is the process of finding a class with a particular name and creating a representation from the associated class file 200 of that class within the memory of the runtime environment 113. For example, creating the run-time constant pool 304, method code 305, and field and method data 306 for the class within the per-class area 303 of the virtual machine memory layout 300. Linking is the process of taking the in-memory representation of the class and combining it with the run-time state of the virtual machine 104 so that the methods of the class can be executed. Initialization is the process of executing the class constructors to set the starting state of the field and method data 306 of the class and/or create class instances on the heap 302 for the initialized class.

The following are examples of loading, linking, and initializing techniques that may be implemented by the virtual machine 104. However, in many embodiments the steps may be interleaved, such that an initial class is loaded, then during linking a second class is loaded to resolve a symbolic reference found in the first class, which in turn causes a third class to be loaded, and so forth. Thus, progress through the stages of loading, linking, and initializing can differ from class to class. Further, some embodiments may delay (perform “lazily”) one or more functions of the loading, linking, and initializing process until the class is actually required. For example, resolution of a method reference may be delayed until a virtual machine instruction invoking the method is executed. Thus, the exact timing of when the steps are performed for each class can vary greatly between implementations.

To begin the loading process, the virtual machine 104 starts up by invoking the class loader 107 which loads an initial class. The technique by which the initial class is specified will vary from embodiment to embodiment. For example, one technique may have the virtual machine 104 accept a command line argument on startup that specifies the initial class.

To load a class, the class loader 107 parses the class file 200 corresponding to the class and determines whether the class file 200 is well-formed (meets the syntactic expectations of the virtual machine 104). If not, the class loader 107 generates an error. For example, in Java the error might be generated in the form of an exception which is thrown to an exception handler for processing. Otherwise, the class loader 107 generates the in-memory representation of the class by allocating the run-time constant pool 304, method code 305, and field and method data 306 for the class within the per-class area 303.

In some embodiments, when the class loader 107 loads a class, the class loader 107 also recursively loads the super-classes of the loaded class. For example, the virtual machine 104 may ensure that the super-classes of a particular class are loaded, linked, and/or initialized before proceeding with the loading, linking and initializing process for the particular class.

During linking, the virtual machine 104 verifies the class, prepares the class, and performs resolution of the symbolic references defined in the run-time constant pool 304 of the class.

To verify the class, the virtual machine 104 checks whether the in-memory representation of the class is structurally correct. For example, the virtual machine 104 may check that each class except the generic class Object has a superclass, check that final classes have no sub-classes and final methods are not overridden, check whether constant pool entries are consistent with one another, check whether the current class has correct access permissions for classes/fields/structures referenced in the constant pool 304, check that the virtual machine 104 code of methods will not cause unexpected behavior (e.g. making sure a jump instruction does not send the virtual machine 104 beyond the end of the method), and so forth. The exact checks performed during verification are dependent on the implementation of the virtual machine 104. In some cases, verification may cause additional classes to be loaded, but does not necessarily require those classes to also be linked before proceeding. For example, assume Class A contains a reference to a static field of Class B. During verification, the virtual machine 104 may check Class B to ensure that the referenced static field actually exists, which might cause loading of Class B, but not necessarily the linking or initializing of Class B. However, in some embodiments, certain verification checks can be delayed until a later phase, such as being checked during resolution of the symbolic references. For example, some embodiments may delay checking the access permissions for symbolic references until those references are being resolved.

To prepare a class, the virtual machine 104 initializes static fields located within the field and method data 306 for the class to default values. In some cases, setting the static fields to default values may not be the same as running a constructor for the class. For example, the verification process may zero out or set the static fields to values that the constructor would expect those fields to have during initialization.

During resolution, the virtual machine 104 dynamically determines concrete memory address from the symbolic references included in the run-time constant pool 304 of the class. To resolve the symbolic references, the virtual machine 104 utilizes the class loader 107 to load the class identified in the symbolic reference (if not already loaded). Once loaded, the virtual machine 104 has knowledge of the memory location within the per-class area 303 of the referenced class and its fields/methods. The virtual machine 104 then replaces the symbolic references with a reference to the concrete memory location of the referenced class, field, or method. In an embodiment, the virtual machine 104 caches resolutions to be reused in case the same class/name/descriptor is encountered when the virtual machine 104 processes another class. For example, in some cases, class A and class B may invoke the same method of class C. Thus, when resolution is performed for class A, that result can be cached and reused during resolution of the same symbolic reference in class B to reduce overhead.

In some embodiments, the step of resolving the symbolic references during linking is optional. For example, an embodiment may perform the symbolic resolution in a “lazy” fashion, delaying the step of resolution until a virtual machine instruction that requires the referenced class/method/field is executed.

During initialization, the virtual machine 104 executes the constructor of the class to set the starting state of that class. For example, initialization may initialize the field and method data 306 for the class and generate/initialize any class instances on the heap 302 created by the constructor. For example, the class file 200 for a class may specify that a particular method is a constructor that is used for setting up the starting state. Thus, during initialization, the virtual machine 104 executes the instructions of that constructor.

In some embodiments, the virtual machine 104 performs resolution on field and method references by initially checking whether the field/method is defined in the referenced class. Otherwise, the virtual machine 104 recursively searches through the super-classes of the referenced class for the referenced field/method until the field/method is located, or the top-level superclass is reached, in which case an error is generated.

3. AN UPDATED TYPE

FIG. 5 illustrates an example of an updated type, in accordance with one or more embodiments. As illustrated, an updated type 502 includes fields 506, methods 514, one or more migration relationships 522, and one or more conversion functions 524. In one or more embodiments, the updated type 502 may include more or fewer components than the components illustrated in FIG. 5. As an example, an updated type may include fields 506 without including methods 514. Alternatively, an updated type may include methods 514 without including fields 506.

In one or more embodiments, each of fields 506 (such as fields 506 a-b) is associated with a same field name 508 but different field types (such as field types 510 a-b). Each field 506 may be referred to as a different version of a “migrated field.”

As an example, an updated type 502 may include two fields with a same field name, fd. One of the two fields may be associated with the field type Date. The other of the two fields may be associated with the field type NewDate. The field type Date may include month and year only. The field type NewDate may include month, date, and year. Example code including the above-described migrated field may be written as follows:

class Account {   Date fd;   @MigratedFrom (Date fd, ConversionFunctionPair)   NewDate fd; }

In the above example, the line beginning with @MigratedFrom may be used to identify a migration relationship 522 between Date fd and NewDate fd. Migration relationships 522 are further described below.

In one or more embodiments, each of methods 514 (such as methods 514 a-b) is associated with a same method name 516. But each method is associated with different sets of parameter types (such as parameter types 518 a-b) and/or different return types (such as return types 520 a-b). A set of parameter types includes one or more parameter types corresponding respectively to one or more parameters that are input to a method. A return type corresponds to a value returned by a method. The term “method descriptor” refers to (a) the method name, (b) the set of parameter types, and (c) the return type associated with a particular method. Each method, which is associated with a same method name but a different method descriptor, may be referred to as a different version of a “migrated method.”

As an example, an updated type includes two methods with a same method name, getAccess. One of the two methods is associated with one parameter of the parameter type byte. The method is further associated with a return type of Date. The other of the two methods is associated with two parameters, of the parameter types char and String respectively. The method is further associated with a return type of NewDate. Example code including the above-described migrated method may be written as follows:

class Account {   Date getAccess(byte arg1) { ... };   @MigratedFrom (Date getAccess(byte arg1),     ParameterConversionFunctionPair,     ReturnConversionFunctionPair)   NewDate getAccess(char arg1, String arg2) { ... }; }

In the above example, the line beginning with @MigratedFrom may be used to identify a migration relationship 522 between Date getAccess(byte arg1) and NewDate getAccess(char arg1, String arg2). Migration relationships 522 are further described below.

In one or more embodiments, additional and/or alternative migrated members may be included in the updated type 502. A member in a type is an inheritable component of the type. A member in a type is inherited by a sub-type, given that there are no access restrictions to the member based on access level modifiers. A member in a type may be directly declared in the body of the type. Additionally or alternatively, a member may be included in the type via inheritance. Examples of members include a field, a method, a nested class, an interface, and an enumerated type. An updated type 502 is a type that includes one or more migrated members.

In one or more embodiments, a migration relationship 522 indicates a sequence in which each version of a migrated member was created. Additionally or alternatively, a migration relationship 522 identifies which particular version of a migrated member was migrated from another version of the migrated member. As an example, a migration relationship may indicate that field 506 b was migrated from field 506 a. Another migration relationship may indicate that method 514 b was migrated from method 514 a.

A migration relationship 522 may be specified in a migration tag that is associated with one or more versions of a migrated member. As described above, example code including a migration tag may be written as follows:

class Account {   Date fd;   @MigratedFrom (Date fd, ConversionFunctionPair)   NewDate fd; }

The line beginning with @MigratedFrom is a migration tag associated with NewDate fd. In the parentheses, the term “Date fd” indicates that NewDate fd was migrated from Date fd. Also in the parentheses, the term “ConversionFunctionPair” identifies a pair of conversion functions. The term “ConversionFunctionPair” may be, for example, a reference to a pair of conversion functions. Conversion functions are further described below.

Also described above, example code including a migration tag may be written as follows:

class Account {   Date getAccess(byte arg1) { ... };   @MigratedFrom (Date getAccess(byte arg1),     ParameterConversionFunctionPair,     ReturnConversionFunctionPair)   NewDate getAccess(char arg1, String arg2) { ... } }

The line beginning with @MigratedFrom is a migration tag associated with NewDate getAccess(char arg1, String arg2). In the parentheses, the term “Date getAccess(byte arg1)” indicates that NewDate getAccess(char arg1, String arg2) was migrated from Date getAccess(byte arg1). Also in the parentheses, the term “ParameterConversionFunctionPair” identifies a pair of conversion functions for parameter types of the migrated method. The term “ReturnConversionFunctionPair” identifies a pair of conversion functions for return types of the migrated method. Conversion functions are further described below.

In an embodiment, a migrated member may be associated with three or more versions. Migration relationships between the different versions may be specified using multiple migration tags. As an example, a migrated member may be a field named fd. The versions associated with the migrated member may be: int fd; Date fd; NewDate fd. One migration tag associated with Date fd may indicate that Date fd was migrated from int fd. Another migration tag associated with NewDate fd may indicate that NewDate fd was migrated from Date fd.

Additionally or alternatively, a migration relationship 522 may be specified in a file and/or database that is separate from the file including the updated type 502. As an example, a separate file may store a version list. The version list may be a sequenced list of versions of a migrated member. As an example, a sequenced list may include: int fd; Date fd; NewDate fd. The sequenced list may indicate that int fd is the earliest version of the migrated field fd; Date fd is a subsequent version of the migrated field fd; and NewDate fd is the current version of the migrated field fd.

In one or more embodiments, a conversion function 524 converts values of a particular set of types into values of another set of types.

As an example, a conversion function may convert a double value into an int value. The double type may be a signed 64-bit two's complement integer, while the int type may be a signed 32-bit two's complement integer. The range of the int type may be −2,147,483,648 to +2,147,483,647. When converting a double value into an int value, the double value is compared to the range of the int type. If the double value is within the range of the int type, then the least significant 32-bits of the double value may be determined as the converted int value. If the double value is above the maximum value of the int type, then the maximum value of the int type (+2,147,483,647) may be determined as the converted int value. If the double value is below the minimum value of the int type, then the minimum value of the int type (−2,147,483,648) may be determined as the converted int value.

As another example, a conversion function may convert an int value into a double value. If the int value is a positive number, then the conversion function may pad thirty-two zeroes in front of the int value. If the int value is a negative number, then the conversion function may pad thirty-two ones in front of the int value. The padded int value may be determined as the converted double value.

A pair of conversion functions includes a projection function and an embedding function. The embedding function performs a reverse conversion as compared to the projection function. The embedding function that converts a value of a first type into a value of a second type. Conversely, the projection function that converts a value of the first type into a value of the second type. However, the embedding function and the projection function may not be exact inverses of each other. The embedding function and the projection function have the following properties, as described below.

Given a first value of the first type, an embedding function may be applied to the first value. A projection function may be applied to the result of the embedding function. The result of this application of the projection function is the first value itself.

Given a second value of the second type, a projection function may be applied to the second value. An embedding function may be applied to the result of the projection function. The result of this application of the embedding function is either the second value itself, or a value similar and/or related to the second value.

The following example illustrates the above properties of an embedding function and a projection function. As an example, an embedding function may convert an int value into a double value. A projection function may convert a double value into an int value.

An initial int value may be a binary number including thirteen zeroes in the most significant bits, followed by nineteen ones in the least significant bits (which is 524,287 in decimal). The embedding function may be applied to the initial int value. Based on the embedding function, thirty-two zeros may be padded in front of the int value. The converted double value may be a binary number including forty-five zeroes in the most significant bits, followed by nineteen ones in the least significant bits (which is 524,287 in decimal). The projection function may be applied to the converted double value. Based on the projection function, the double value may be compared to the range of the int type. Since the double value is within the range, the least significant 32-bits of the double value may be determined as the converted int value. The converted int value may be a binary number including thirteen zeroes in the most significant bits, followed by nineteen ones in the least significant bits (which is 524,287 in decimal). Hence, the output of the projection function is the same as the initial int value.

Conversely, an initial double value may be a binary number including thirty zeroes in the most significant bits, and thirty-four ones in the least significant bits (which is 17,179,869,184 in decimal). The projection function may be applied to the initial double value. Based on the projection function, the double value may be compared to the range of the int type. Since the double value is above the maximum value of the int type, the maximum value of the int type may be determined as the converted int value. The converted int value may be one zero in the most significant bit, followed by thirty-one ones in the least significant bits (which is 2,147,483,647 in decimal). The embedding function may be applied to the converted int value. Based on the embedding function, thirty-two zeroes may be padded in front of the int value. The converted double value may be a binary number including thirty-three zeroes in the most significant bits, followed by thirty-one ones in the least significant bits (which is 2,147,483,647 in decimal). The output of the embedding function is different from the initial double value. Hence, the output of the embedding function is not the same as the initial double value. The output of the embedding function is related to the initial double value, in that (a) the initial double value exceeds the maximum value of the int type, and (b) the output of the embedding function is the maximum value of the int type.

In an embodiment, a conversion function converts a set of values of a particular set of types into another set of values of another set of types. The number of values in the input of a conversion function may be the same as or different from the number of values in the output of a conversion function. As an example, a conversion function may convert (a) an int value into (b) a double value and a String value. If the int value is a positive number, thirty-two zeroes may be padded in front of the int value to obtain the converted double value. If the int value is a negative number, thirty-two ones may be padded in front of the int value to obtain the converted double value. Additionally, the conversion function may set the String value to null. The output of the conversion function includes both the converted double value and the String value that is set to null.

In an embodiment, a chain of conversion functions may be used to convert a value of one type into a value of another type. As described above, a migrated member may be associated with three or more versions. One conversion function may convert between two particular versions. Another conversion function may convert between two other versions. The two conversion functions may be chained together. As an example, a migrated member may be a field named fd. The versions associated with the migrated member may be: int fd; Date fd; NewDate fd. A first conversion function may convert an int value into a Date value. A second conversion function may convert a Date value into a NewDate value. To convert a particular int value into a NewDate value, the first conversion function may be applied to the particular int value, and the second conversion function may be applied to the result of the first conversion function. Hence, the first and second conversion functions are chained together to convert an int value into a NewDate value.

4. COMPILING AN UPDATED TYPE

FIG. 6 illustrates a set of operations for compiling an updated type comprising an migrated member, in accordance with one or more embodiments. One or more operations illustrated in FIG. 6 may be modified, rearranged, or omitted all together. Accordingly, the particular sequence of operations illustrated in FIG. 6 should not be construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a source file, an updated type including multiple versions of a migrated member (Operation 602). Identifying the updated type may be performed as part of a process to compile the updated type.

In an embodiment, the updated type includes multiple versions of a migrated field, each associated with a same field name but different field types, as explained above with reference to fields 506 in FIG. 5. Additionally or alternatively, the updated type includes multiple versions of a migrated method, each associated with a same method name but different parameter types and/or different return types, as explained above with reference to methods 514 in FIG. 5.

One or more embodiments include identifying one or more migration relationships between the versions of the migrated member (Operation 604). A migration relationship is identified based on one or more migration tags that are associated with one or more versions of the migrated member. The migration tag indicates a sequence in which the versions of the migrated member were generated. Additionally or alternatively, the migration relationship is identified from a separate file and/or database.

One or more embodiments include determining whether the migrations are valid (Operation 606).

In an embodiment, a migration may be invalid if a version of a migrated member was associated with a security setting that prohibited migration. For example, a version of the migrated member may be marked with an “Unmigratable” tag.

In an embodiment, a migration may be invalid if different versions of a migrated member are associated with conflicting migration relationships. As an example, an updated type may include two fields with a same field name, fd. One of the two fields may be associated with the type Date. The other of the two fields may be associated with the type NewDate. The field NewDate fd may be associated with a migration tag that indicates NewDate fd was migrated from Date fd. Meanwhile, the field Date fd may be associated with a migration tag that indicates Date fd was migrated from NewDate fd. The two migration tags indicate conflicting migration relationships. The migration is invalid.

If the migration is not valid, then an error is returned (Operation 608). The source file is not successfully compiled.

One or more embodiments include identifying one or more pairs of conversion functions (Operation 610). The pairs of conversion functions may be used to convert between the different versions of the migrated member. References to the conversion functions are identified from the source file including the updated type. As an example, references to conversion functions may be identified from a migration tag associated with a version of a migrated member. Based on the references to the conversion functions, the conversion functions may be identified from the source file itself or a different file.

One or more embodiments include storing, in a class file, a set of member structures corresponding to the versions of the migrated member (Operation 612).

In an embodiment, the updated type includes multiple fields, each associated with a same field name. The multiple fields are different versions of a migrated field. Compilation of the multiple fields associated with the same field name does not cause a generation of an error. Rather, a set of field structures, corresponding to the versions of the migrated field, are stored in the compiled class file.

In an embodiment, the updated type includes multiple methods, each associated with a same method name but different parameter types and/or return types. The multiple methods are different versions of a migrated method. A set of method structures, corresponding to the versions of the migrated method, are stored in the compiled class file. As an example, an updated type may include multiple methods associated with a same method signature. The term “method signature” refers to (a) the method name of a method, and (b) the set of parameter types associated with the method. The methods may be associated with different return types. The methods are different versions of a migrated method. Compilation of the multiple methods associated with the same method signature does not cause a generation of an error. Rather, a set of method structures, corresponding to the versions of the migrated method, may be stored in a compiled class file.

One or more embodiments include storing, in the class file, the migration relationships (Operation 614). The migration relationships may be stored as migration tags in the class file. Additionally or alternatively, the migration relationships may be stored as a version list in the class file. Additionally or alternatively, other formats may be used for storing the migration relationships.

One or more embodiments include storing, in the class file, references to the pairs of conversion functions (Operation 616). The references to the pairs of conversion functions may be stored in a constant pool table of the class file. Additionally or alternatively, other formats may be used for storing the references to the pairs of conversion functions.

In an embodiment, the conversion functions themselves may be stored in the class file. In another embodiment, the conversion functions are stored in a separate file and/or database.

5. ACCESSING A MIGRATED FIELD

FIGS. 7A-B illustrate a set of operations for fetching and returning a value of a migrated field as a value of a particular type, in accordance with one or more embodiments. One or more operations illustrated in FIGS. 7A-B may be modified, rearranged, or omitted all together. Accordingly, the particular sequence of operations illustrated in FIGS. 7A-B should not be construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a class file, one or more instructions to fetch and return (a) a value of a migrated field corresponding to an object as (b) a value of a particular type (Operation 704). Identifying the instructions in the class file may be performed as part of a process to execute the class file. In an embodiment, the instructions are identified from the body of a main method. The main method is a method that is initially called by the runtime environment. In another embodiment, the instructions are identified from the body of another method that is called by the main method.

Example code may be written as follows:

class Main {   public static void main(String[ ] args) {     Account myaccount = new Account( );     int myfd = myaccount.fd;   } }

The example code may be compiled into bytecode, which may be stored in a class file. The instructions to fetch and return (a) a value of a migrated field corresponding to an object as (b) a value of a particular type include the bytecode corresponding to the line int myfd=myaccount.fd. The bytecode corresponding to the line int myfd=myaccount.fd may include, for example, a getfield instruction. Based on the line int myfd=myaccount.fd, a value of a particular field, fd, corresponding to the object, myaccount, is to be fetched. The value of the particular field, fd, is to be returned as a value of the particular type, int. The particular field, fd, may subsequently be determined as a migrated field by identifying an updated type that includes the particular field, as further described below with reference to Operation 706.

One or more embodiments include identifying multiple versions of the migrated field, each associated with a same field name but different field types (Operation 706).

An updated type including the migrated field is identified. The updated type is identified based on a declaration of the object corresponding to the migrated field. The declaration of the object indicates a type associated with the object. The type associated with the object is the updated type including the migrated field.

Referring to the example code above, the migrated field, fd, corresponds to the object, myaccount. A declaration of myaccount is the line Account myaccount=new Account( ). Based on the declaration of myaccount, the type of myaccount is determined as Account. Account is the updated type including the migrated field, fd.

If the updated type is not yet loaded, then the updated type is loaded into a per-class area of the runtime environment.

After loading the updated type, multiple versions of the migrated field are identified from the updated type. Each version of the migrated field is associated with a same field name but different field types. As an example, Account may be an updated type including the migrated field, fd. The updated type Account may include the following code:

class Account {   int fd;   @MigratedFrom (int fd, ConversionFunctionPair)   long fd; }

The example code for the updated type, Account, includes multiple fields associated with the field name, fd. One fd field is associated with the field type int. Another fd field is associated with the field type long. The two fd fields are different versions of a migrated field. The versions of the migrated field are identified from the updated type Account.

One or more embodiments include determining one or more migration relationships between the versions of the migrated field (Operation 708). The migration relationships between the versions of the migrated field are stored as part of the update type. The migration relationships are identified from the updated type. Additionally or alternatively, the migration relationships are identified based on references stored as part of the updated type.

Based on the migration relationships, a sequence in which the versions of the migrated field were generated is determined. A current version of the migrated field is determined.

One or more embodiments include determining whether the particular type, specified in the instructions identified at Operation 704, is the same as a field type associated with any of the versions of the migrated field (Operation 710). The particular type, specified in the instructions identified at Operation 704, is compared to each of the field types associated with the versions of the migrated field.

If the particular type is not the same as a field type associated with any of the versions of the migrated field, then an error is returned (Operation 712). The class file is not successfully executed.

One or more embodiments include fetching a value associated with a current version of the migrated field corresponding to the object (Operation 714). As discussed above with reference to Operation 708, the sequence in which the versions of the migrated field were generated is determined based on the migration relationships. The version that was last generated is the current version of the migrated field.

As described above, instructions to fetch a value of the migrated field corresponding to the object are identified at Operation 704. In an embodiment, the instructions include a symbolic reference to the migrated field. During linkage of the class file, the symbolic reference to the migrated field is resolved as a direct reference to the current version of the migrated field.

As an example, a class file may include a symbolic reference to a migrated field, fd. An updated type, Account, including the migrated field may include two versions of fd: int fd and long fd. Based on a migration relationship between the two versions, long fd may be identified as a current version of the migrated field. The symbolic reference to fd may be resolved as a direct reference to long fd.

Based on the resolved direct reference to the current version of the migrated field, attributes of the current version of the migrated field are identified from the updated type. One of the attributes includes an offset, with respect to a memory location of an object of the updated type, associated with the current version of the migrated field. The sum of the memory location of the object of the updated type and the offset is a memory location of a value of the current version of the migrated field corresponding to the object.

During execution of the class file, a memory location of the object, corresponding to the migrated field, is identified. A sum of (a) the memory location of the object and (b) the offset is computed. As described above, the sum is determined as a memory location of a value of the current version of the migrated field corresponding to the object. A value stored at the determined memory location is fetched.

Additional and/or alternative methods for determining a memory location of a value of the current version of the migrated field may be used. A value stored at the determined memory location is fetched.

One or more embodiments include determining whether the particular type, specified in the instructions identified at Operation 704, is the same as the field type associated with the current version of the migrated field (Operation 716). As described above, based on the resolved direct reference to the current version of the migrated field, attributes of the current version of the migrated field are identified from the updated type. One of the attributes includes a field type associated with the current version of the migrated field. Hence, the field type associated with the current version of the migrated field is determined from the updated type. Additional and/or alternative methods for determining a field type associated with the current version of the migrated field may be used.

The particular type, specified in the instructions identified at Operation 704, is compared to the field type associated with the current version of the migrated field.

If the particular type, specified in the instructions identified at Operation 704, is the same as the field type associated with the current version of the migrated field, then the fetched value is returned (Operation 718). The fetched value is stored as the value of the particular type, as specified in the instructions identified at Operation 704.

One or more embodiments include identifying one or more conversion functions that convert (a) a value of the field type associated with the current version of the migrated field into (b) a value of the particular type (Operation 720). The conversion function may be identified from the updated type, the class file identified at Operation 704, a separate file, and/or a separate database. As an example, an updated type may include a migrated field and a migration tag associated with the migrated field. The migration tag may identify a reference to a conversion function. The reference may be an index to a constant pool entry in the updated type. The conversion function may be identified from the constant pool table based on the index.

One or more embodiments include applying the conversion functions to convert (a) the fetched value into (b) a value of the particular type (Operation 722).

A single conversion function may be used to convert (a) the fetched value into (b) a value of the particular type, as described above with reference to conversion functions 524 of FIG. 5. Alternatively, multiple conversion functions may be chained together to convert (a) the fetched value into (b) a value of the particular type, as described above with reference to conversion functions 524 of FIG. 5.

One or more embodiments include returning the converted value (Operation 724). The converted value is stored as the value of the particular type, as specified in the instructions identified at Operation 704.

FIGS. 8A-B illustrate a set of operations for storing a value of a particular type as a value of a migrated field, in accordance with one or more embodiments. One or more operations illustrated in FIGS. 8A-B may be modified, rearranged, or omitted all together. Accordingly, the particular sequence of operations illustrated in FIGS. 8A-B should not be construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a class file, one or more instructions to store (a) a value of a particular type as (b) a value of a migrated field corresponding to an object (Operation 804). Identifying the instructions in the class file may be performed as part of a process to execute the class file. Further descriptions regarding the operation of identifying instructions are included above with reference to Operation 704.

Example code may be written as follows:

class Main {   public static void main(String[ ] args) {     Account myaccount = new Account( );     int myfd = 10;     myaccount.fd = myfd;   } }

The example code may be compiled into bytecode, which may be stored in a class file. The instructions to store (a) a value of a particular type as (b) a value of a migrated field corresponding to an object include the bytecode corresponding to the line myaccount.fd=myfd. The bytecode corresponding to the line myaccount.fd=myfd may include, for example, a putfield instruction. Based on the line myaccount.fd=myfd, a value of the particular type, int, is to be stored as a value of the particular field, fd, corresponding to the object, myaccount. The particular field, fd, may subsequently be determined as a migrated field by identifying an updated type that includes the particular field, as further described below with reference to Operation 806.

One or more embodiments include identifying multiple versions of the migrated field, each associated with a same field name but different field types (Operation 806). Further descriptions regarding the operation of identifying multiple versions of the migrated field are included above with reference to Operation 706.

One or more embodiments include determining one or more migration relationships between the versions of the migrated field (Operation 808). Further descriptions regarding the operation of determining migration relationships are included above with reference to Operation 708.

One or more embodiments include determining whether the particular type, specified in the instructions identified at Operation 804, is the same as a field type associated with any of the versions of the migrated field (Operation 810). The particular type, specified in the instructions identified at Operation 804, is compared to each of the field types associated with the versions of the migrated field.

If the particular type is not the same as a field type associated with any of the versions of the migrated field, then an error is returned (Operation 812). The class file is not successfully executed.

One or more embodiments include determining whether the particular type, specified in the instructions identified at Operation 804, is the same as the field type associated with the current version of the migrated field (Operation 814).

As described above, instructions to store a value of the particular type as a value of the migrated field corresponding to the object are identified at Operation 804. In an embodiment, the instructions include a symbolic reference to the migrated field. During linkage of the class file, the symbolic reference to the migrated field is resolved as a direct reference to the current version of the migrated field. Further descriptions regarding the operation of resolving a symbolic reference as a direct reference are included above with reference to Operation 714.

Based on the resolved direct reference to the current version of the migrated field, attributes of the current version of the migrated field are identified from the updated type. One of the attributes includes a field type associated with the current version of the migrated field. Hence, the field type associated with the current version of the migrated field is determined from the updated type. Additional and/or alternative methods for determining a field type associated with the current version of the migrated field may be used. Further descriptions regarding determining a field type associated with the current version of the migrated field are included above with reference to Operation 716.

The particular type, specified in the instructions identified at Operation 804, is compared to the field type associated with the current version of the migrated field.

If the particular type is the same as the field type associated with the current version of the migrated field, then the value of the particular type, specified in the instructions identified at Operation 804, is stored as a value of the current version of the migrated field corresponding to the object (Operation 816).

As described above, based on the resolved direct reference to the current version of the migrated field, attributes of the current version of the migrated field are identified from the updated type. One of the attributes includes an offset, with respect to a memory location of an object of the updated type, associated with the current version of the migrated field. The sum of the memory location of the object of the updated type and the offset is a memory location of a value of the current version of the migrated field corresponding to the object.

During execution of the class file, a memory location of the object, corresponding to the migrated field, is identified. The sum of (a) the memory location of the object and (b) the offset is computed. As described above, the sum is determined as a memory location of a value of the current version of the migrated field corresponding to the object. The value of the particular type, specified in the instructions identified at Operation 804, is stored into the determined memory location.

Additional and/or alternative methods for determining a memory location of a value of the current version of the migrated field may be used. Further descriptions regarding determining a memory location of a value of the current version of the migrated field are included above with reference to Operation 714. The value of the particular type, specified in the instructions identified at Operation 804, is stored into the determined memory location.

One or more embodiments include identifying one or more conversion functions that convert (a) a value of the particular type into (b) a value of the field type associated with the current version of the migrated field (Operation 818). Further descriptions regarding the operation of identifying conversion functions are included above with reference to Operation 720.

One or more embodiments include applying the conversion functions to convert (a) the value of the particular type, specified in the instructions identified at Operation 804, into (b) a value of the field type associated with the current version of the migrated field (Operation 820). Further descriptions regarding the operation of applying conversion functions are included above with reference to Operation 722.

One or more embodiments include storing the converted value as a value of the current version of the migrated field corresponding to the object (Operation 822). As described above with reference to Operation 816, a memory location of a value of the current version of the migrated field corresponding to the object is determined. The converted value, output from Operation 820, is stored into the determined memory location.

6. ACCESSING A MIGRATED METHOD

FIGS. 9A-B illustrate a set of operations for invoking a method using values of a particular set of types as arguments to the method, in accordance with one or more embodiments. One or more operations illustrated in FIGS. 9A-B may be modified, rearranged, or omitted all together. Accordingly, the particular sequence of operations illustrated in FIGS. 9A-B should not be construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a class file, one or more instructions to invoke a migrated method, corresponding to an object, using a set of arguments (Operation 904). Identifying the instructions in the class file may be performed as part of a process to execute the class file. Further descriptions regarding the operation of identifying instructions are included above with reference to Operation 704.

Example code may be written as follows:

class Main {   public static void main(String[ ] args) {     Account myaccount = new Account( );     byte myarg1 = 3;     int dt = myaccount.getAccess(myarg1);   } }

The example code may be compiled into bytecode, which may be stored in a class file. The instructions to invoke the migrated method, corresponding to the object, using the set of arguments include the bytecode corresponding to the line int dt=myaccount.getAccess(myarg1). The bytecode corresponding to the line int dt=myaccount.getAccess(myarg1) may include, for example, a load instruction and an invoke instruction. Based on the line int dt=myaccount.getAccess(myarg1), an argument of a particular type, byte, is loaded onto an operand stack. Further, a particular method, getAccess, corresponding to the object, myaccount, is executed using the loaded value as an argument. The particular method, getAccess, may subsequently be determined as a migrated method by identifying an updated type that includes the particular method, as further described below with reference to Operation 906.

One or more embodiments include identifying multiple versions of the migrated method, each associated with a same method name but different sets of parameter types (Operation 906).

An updated type including the migrated method is identified. The updated type is identified based on a declaration of the object corresponding to the migrated method. The declaration of the object indicates a type associated with the object. The type associated with the object is the updated type including the migrated method.

Referring to the example code above, the migrated method, getAccess, corresponds to the object, myaccount. A declaration of myaccount is the line Account myaccount=new Account( ). Based on the declaration of myaccount, the type of myaccount is determined as Account. Account is the updated type including the migrated method, getAccess.

If the updated type is not yet loaded, then the updated type is loaded into a per-class area of the runtime environment.

After loading the updated type, multiple versions of the migrated method are identified from the updated type. Each version of the migrated method is associated with a same method name but different sets of parameter types and/or different return types. As an example, Account may be an updated type including the migrated method, getAccess. The updated type Account may include the following code:

class Account {   int getAccess (byte arg1);   @MigratedFrom (int getAccess(byte arg1),     ParameterConversionFunctionPair)   int getAccess (char arg1, String arg2); }

The example code for the updated type, Account, includes multiple methods associated with the method name, getAccess. One getAccess method is associated with the set of parameter types, (byte). Another getAccess method is associated with the set of parameter types, (char, String). The two getAccess methods are different versions of a migrated method. The versions of the migrated method are identified from the updated type Account.

One or more embodiments include determining one or more migration relationships between the versions of the migrated method (Operation 908). Further descriptions regarding the operation of determining migration relationships are included above with reference to Operation 708.

One or more embodiments include determining whether a set of types, associated with the set of arguments, is the same as a set of parameter types associated with any of the versions of the migrated method (Operation 910). The set of types, associated with the set of arguments, is compared to each of the set of parameter types associated with the versions of the migrated method.

If the set of types, associated with the set of arguments, is not the same as a set of parameter types associated with any of the versions of the migrated method, then an error is returned (Operation 912). The class file is not successfully executed.

One or more embodiments include determining whether the set of types, associated with the set of arguments, is the same as the set of parameter types associated with the current version of the migrated method (Operation 914). Based on the migration relationships determined at Operation 908, the sequence in which the versions of the migrated method were created is determined. The version that was last created is the current version of the migrated method.

The set of types, associated with the set of arguments specified in the instructions identified at Operation 904, is compared to the set of parameter types associated with the current version of the migrated method.

If the set of types, associated with the set of arguments, is the same as the set of parameter types associated with the current version of the migrated method, then the current version of the migrated method is executed using the set of arguments specified in the instructions identified at Operation 904 (Operation 916).

As described above, instructions to invoke the migrated method, corresponding to the object, are identified at Operation 904. In an embodiment, the instructions include a symbolic reference to the migrated method. During linkage of the class file, the symbolic reference to the migrated method is resolved as a direct reference to the current version of the migrated method. The direct reference to the current version of the migrated method is the memory location of the first instruction of the current version of the migrated method.

During execution of the class file, the set of arguments, specified in the instructions identified at Operation 904, are loaded onto an operand stack. The program counter (PC), which references the next instruction to be executed, is moved to the direct reference to the current version of the migrated method. Based on the PC, the current version of the migrated method is executed. Further, the loaded values on the operand stack are used as arguments for the current version of the migrated method.

Additional and/or alternative methods for identifying the memory location associated with the current version of the migrated method may be used. Additional and/or alternative methods for inputting and/or loading the set of arguments for the current version of the migrated method may be used.

One or more embodiments include identifying one or more conversion functions that convert (a) values of the set of types associated with the set of arguments into (b) values of the set of parameter types associated with the current version of the migrated method (Operation 918). Further descriptions relating to identifying conversion functions are included above with reference to Operation 720.

In an embodiment, a separate conversion function is used for each parameter of a migrated method. As an example, a migrated method may have the parameters arg1 and arg2. One version of the migrated method may specify that arg1 is of type int, and arg2 is of type char. Another version of the migrated method may specify that arg1 is of type double, and arg2 is of type long. Two separate conversion functions may be associated with arg1 and arg2, respectively. Specifically, a first conversion function, associated with arg1, may convert an int value into a double value. A second conversion function, associated with arg2, may convert a char value into a long value.

In an embodiment, a single conversion function is used for a set of parameters of a migrated method. As an example, a migrated method may have the parameters arg1 and arg2. One version of the migrated method may specify that arg1 is of type int, and arg2 is of type char. Another version of the migrated method may specify that arg1 is of type double, and arg2 is of type long. A single conversion function may convert a set of values comprising an int value and a char value into another set of values comprising a double value and a long value.

One or more embodiments include applying the conversion functions to convert (a) the set of arguments into (b) values of the set of parameter types associated with the current version of the migrated method (Operation 920). Further descriptions relating to applying conversion functions are included above with reference to Operation 722.

In an embodiment, as described above, a separate conversion function is used for each parameter of a migrated method. Hence, a separate conversion function is applied to each of the set of arguments. The converted values, output from each conversion function, are input as arguments to the current version of the migrated method.

In an embodiment, as described above, a single conversion function is used for a set of parameters of a migrated method. Hence, the single conversion function is applied to the set of arguments. The converted values, output from the single conversion function, are input as arguments to the current version of the migrated method.

One or more embodiments include executing the current version of the migrated method using the converted values (Operation 922). As described above with reference to Operation 916, a memory location associated with the current version of the migrated method is identified. The current version of the migrated method is executed from the identified memory location. Further, the converted values, output from Operation 920, are loaded as arguments for the current version of the migrated method. The current version of the migrated method is executed using the converted values.

FIGS. 10A-B illustrate a set of operations for returning a value from a method as a value of a particular type, in accordance with one or more embodiments. One or more operations illustrated in FIGS. 10A-B may be modified, rearranged, or omitted all together. Accordingly, the particular sequence of operations illustrated in FIGS. 10A-B should not be construed as limiting the scope of one or more embodiments.

One or more embodiments include identifying, in a class file, instructions to invoke a migrated method, corresponding to an object, and to return a value from the migrated method as a value of a particular type (Operation 1004). Identifying the instructions in the class file may be performed as part of a process to execute the class file. Further descriptions regarding the operation of identifying instructions are included above with reference to Operation 704.

Example code may be written as follows:

class Main {   public static void main(String[ ] args) {     Account myaccount = new Account( );     byte myarg1 = 3;     int dt = myaccount.getAccess(myarg1);   } }

The example code may be compiled into bytecode, which may be stored in a class file. The instructions to invoke the migrated method, corresponding to the object, and to return a value from the migrated method as a value of the particular type include the bytecode corresponding to the line int dt=myaccount.getAccess(myarg1). The bytecode corresponding to the line int dt=myaccount.getAccess(myarg1) may include, for example, an invoke instruction and a store instruction. Based on the line int dt=myaccount.getAccess(myarg1), a particular method, getAccess, corresponding to the object, myaccount, is executed. A value returned from the particular method is stored as a value of the particular type, int. The particular method, getAccess, may subsequently be determined as a migrated method by identifying an updated type that includes the particular method, as further described below with reference to Operation 1006.

One or more embodiments include identifying multiple versions of the migrated method, each associated with a same method name but different return types (Operation 1006). Further descriptions relating to identifying an updated type including the migrated method are included above with reference to Operation 906. Multiple versions of the migrated method are identified from the updated type. Each version of the migrated method is associated with a same method name but different sets of parameter types and/or different return types. As an example, Account may be an updated type including the migrated method, getAccess. The updated type Account may include the following code:

class Account {   int getAccess (byte arg1);   @MigratedFrom (int getAccess(byte arg1),     ReturnConversionFunctionPair)   long getAccess (byte arg1); }

The example code for the updated type, Account, includes multiple methods associated with the method name, getAccess. One getAccess method is associated with the return type, int. Another getAccess method is associated with the return type, long. The two getAccess methods are different versions of a migrated method. The versions of the migrated method are identified from the updated type Account.

One or more embodiments include determining one or more migration relationships between the versions of the migrated methods (Operation 1008). Further descriptions regarding the operation of determining migration relationships are included above with reference to Operation 708.

One or more embodiments include determining whether the particular type, specified in the instructions identified at Operation 1004, is the same as a return type associated with any of the versions of the migrated method (Operation 1010). The particular type, specified in the instructions identified at Operation 1004, is compared to each of the return types associated with the versions of the migrated method.

If the particular type, specified in the instructions identified at Operation 1004, is not the same as a return type associated with any of the versions of the migrated method, then an error is returned (Operation 1012). The class file is not successfully executed.

One or more embodiments include executing the current version of the migrated method (Operation 1014).

As described above with reference to Operation 916, a memory location associated with the current version of the migrated method is identified. The current version of the migrated method is executed from the identified memory location.

One or more embodiments include determining whether the particular type, specified in the instructions identified at Operation 1004, is the same as the return type associated with the current version of the migrated method (Operation 1016). The particular type, specified in the instructions identified at Operation 1004, is compared to the return type associated with the current version of the migrated method.

If the particular type, specified in the instructions identified at Operation 1004, is the same as the return type associated with the current version of the migrated method, then a value is returned from the current version of the migrated method (Operation 1018). The value returned from the current version of the migrated method is returned as a value of the particular type, as specified in the instructions identified at Operation 1004.

One or more embodiments include identifying one or more conversion functions that convert (a) a value of the return type associated with the current version of the migrated method into (b) a value of the particular type (Operation 1020). Further descriptions relating to identifying conversion functions are included above with reference to Operation 720.

One or more embodiments include applying the conversion functions to convert (a) a value returned from the current version of the migrated method into (b) a value of the particular type (Operation 1022). Further descriptions relating to applying conversion functions are included above with reference to Operation 722.

One or more embodiments include returning the converted value (Operation 1024). The converted value is returned as a value of the particular type, as specified in the instructions identified at Operation 1004.

7. EXAMPLE EMBODIMENTS Example 1: Storing a Value of a Particular Type as a Value of a Migrated Field

A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, causes performance of operations comprising:

-   identifying one or more instructions to store (a) a first value of a     first type as (b) a second value of a first field corresponding to a     particular object, wherein the first field is in an updated type and     referenced by a particular field name; -   determining that the first field is associated with a second type     different than the first type; -   responsive to determining that the updated type comprises a second     field associated with (a) the particular field name and (b) the     first type: -   executing a conversion function to convert the first value of the     first type into a third value of the second type; and -   storing the third value as the second value of the first field.

Example 2

The medium of EXAMPLE 1, wherein executing the conversion function is further responsive to: determining that the first field was migrated to the second field.

Example 3: Compiling an Updated Type Including a Migrated Method

A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising:

-   identifying, in a set of source code, a first declaration for a     first method associated with (a) a particular method name, and (b) a     first set of parameter types, the first set of parameter types     comprising one or more parameter types; -   identifying, in the set of source code, a second declaration for a     second method associated with (a) the particular method name,     and (b) a second set of parameter types different than the first set     of parameter types; -   determining that the first method was migrated to the second method; -   storing, in a set of compiled code, a first method structure     associated with (a) the particular method name, and (b) the first     set of parameter types; -   storing, in the set of compiled code, a second method structure     associated with (a) the particular method name, and (b) the second     set of parameter types; and -   storing, in the set of compiled code, a migration relationship     indicating that the first method was migrated to the second method.

Example 4

The medium of EXAMPLE 3, wherein the operations further comprise: identifying, in the set of source code, a first reference to a conversion function whose effect is to convert one or more values of the first set of parameter types into one or more values of the second set of parameter types; and

-   storing, in the set of compiled code, a second reference to the     conversion function.

Example 5

The medium of EXAMPLE 3, wherein the operations further comprise: identifying, in the set of source code, a first reference to a conversion function that converts a value of a first parameter type, of the first set of parameter types, into a value of a second parameter type, of the second set of parameter types; and

-   storing, in the set of compiled code, a second reference to the     conversion function.

Example 6

The medium of EXAMPLE 3, wherein:

-   the first method is further associated with a first return type; -   the second method is further associated with a second return type     different than the first return type; -   the first method structure is further associated with the first     return type; -   the second method structure is further associated with the second     return type; -   the operations further comprise:     -   identifying, in the set of source code:         -   a first reference to a first conversion function whose             effect is to convert one or more values of the first set of             parameter types into one or more values of the second set of             parameter types;         -   a second reference to a second conversion function that             converts a value of the first return type into a value of             the second return type;     -   storing, in the set of compiled code:         -   a third reference to the first conversion function; and         -   a fourth reference to the second conversion function.

Example 7: Compiling an Updated Type Including a Migrated Method

A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising:

-   identifying, in a set of source code, a first declaration for a     first method associated with (a) a particular method name, and (b) a     first return type; -   identifying, in the set of source code, a second declaration for a     second method associated with (a) the particular method name,     and (b) a second return type different than the first return type; -   determining that the first method was migrated to the second method; -   storing, in a set of compiled code, a first method structure     associated with (a) the particular method name, and (b) the first     return type; -   storing, in the set of compiled code, a second method structure     associated with (a) the particular method name, and (b) the second     return type; and -   storing, in the set of compiled code, a migration relationship     indicating that the first method was migrated to the second method.

Example 8

The medium of EXAMPLE 7, wherein the operations further comprise:

-   identifying, in the set of source code, a first reference to a     conversion function that converts a value of the first return type     into a value of the second return type; and -   storing, in the set of compiled code, a second reference to the     conversion function.

Example 9

The medium of EXAMPLE 7, wherein the operations further comprise: identifying, in the set of source code, a first reference to a conversion function that converts a value of the second return type into a value of the first return type; and storing, in the set of compiled code, a second reference to the conversion function.

Example 10: Compiling an Updated Type Including a Migrated Method

A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising:

-   identifying, in a set of source code, a first declaration for a     first method associated with (a) a particular method name, (b) a set     of one or more parameter types, and (c) a first return type; -   identifying, in the set of source code, a second declaration for a     second method associated with (a) the particular method name, (b)     the set of parameter types, and (c) a second return type different     than the first return type; -   storing, in a set of compiled code, a first method structure     associated with (a) the particular method name, (b) the set of     parameter types, and (c) the first return type; and -   storing, in the set of compiled code, a second method structure     associated with (a) the particular method name, (b) the set of     parameter types, and (c) the second return type.

Example 11

The medium of EXAMPLE 10, wherein the operations further comprise:

refraining from generating a compilation error, during compilation of the set of source code to generate the set of compiled code, based on the set of source code comprising both the first declaration and the second declaration.

Example 12: Invoking a Migrated Method Using Arguments of a Particular Set of Types

A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising:

-   identifying one or more instructions to invoke a particular method     corresponding to a particular object, using values corresponding to     a particular set of types as arguments, wherein the particular     method is in an updated type and referenced by a particular method     name; -   determining that the updated type comprises:     -   a first method associated with the particular method name and a         first set of parameter types; and     -   a second method associated with the particular method name and a         second set of parameter types different than the first set of         parameter types; -   responsive to determining that the particular set of types is same     as the first set of parameter types:     -   executing a conversion function to convert the values         corresponding to the particular set of types into values         corresponding to the second set of parameter types; and     -   invoking the second method using the values corresponding to the         second set of parameter types as the arguments.

Example 13

The medium of EXAMPLE 12, wherein executing the conversion function is further responsive to determining that the first method was migrated to the second method.

Example 14: Returning a Value from a Migrated Method as a Value of a Particular Type

A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising:

-   identifying one or more instructions to return a first value from a     particular method as a second value of a particular type, wherein     the particular method is in an updated type and referenced by a     particular method name; -   determining that the updated type comprises:     -   a first method associated with the particular method name and a         first return type; and     -   a second method associated with the particular method name and a         second return type different than the first return type; -   responsive to determining that the particular type is same as the     first return type:     -   identifying a third value returned from the second method;     -   executing a conversion function to convert the third value into         a fourth value of the particular type; and     -   returning the fourth value as the second value of the particular         type.

Example 15

The medium of EXAMPLE 14, wherein executing the conversion function is further responsive to determining that the first method was migrated to the second method.

Example 16

The medium of EXAMPLE 14, wherein the one or more instructions comprise an instruction to invoke the particular method.

8. MISCELLANEOUS; EXTENSIONS

Embodiments are directed to a system with one or more devices that include a hardware processor and that are configured to perform any of the operations described herein and/or recited in any of the claims below.

In an embodiment, a non-transitory computer readable storage medium comprises instructions which, when executed by one or more hardware processors, causes performance of any of the operations described herein and/or recited in any of the claims.

Any combination of the features and functionalities described herein may be used in accordance with one or more embodiments. In the foregoing specification, embodiments have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the invention, and what is intended by the applicants to be the scope of the invention, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction.

9. HARDWARE OVERVIEW

According to one embodiment, the techniques described herein are implemented by one or more special-purpose computing devices. The special-purpose computing devices may be hard-wired to perform the techniques, or may include digital electronic devices such as one or more application-specific integrated circuits (ASICs) or field programmable gate arrays (FPGAs) that are persistently programmed to perform the techniques, or may include one or more general purpose hardware processors programmed to perform the techniques pursuant to program instructions in firmware, memory, other storage, or a combination. Such special-purpose computing devices may also combine custom hard-wired logic, ASICs, or FPGAs with custom programming to accomplish the techniques. The special-purpose computing devices may be desktop computer systems, portable computer systems, handheld devices, networking devices or any other device that incorporates hard-wired and/or program logic to implement the techniques.

For example, FIG. 11 is a block diagram that illustrates a computer system 1100 upon which an embodiment of the invention may be implemented. Computer system 1100 includes a bus 1102 or other communication mechanism for communicating information, and a hardware processor 1104 coupled with bus 1102 for processing information. Hardware processor 1104 may be, for example, a general purpose microprocessor.

Computer system 1100 also includes a main memory 1106, such as a random access memory (RAM) or other dynamic storage device, coupled to bus 1102 for storing information and instructions to be executed by processor 1104. Main memory 1106 also may be used for storing temporary variables or other intermediate information during execution of instructions to be executed by processor 1104. Such instructions, when stored in non-transitory storage media accessible to processor 1104, render computer system 1100 into a special-purpose machine that is customized to perform the operations specified in the instructions.

Computer system 1100 further includes a read only memory (ROM) 1108 or other static storage device coupled to bus 1102 for storing static information and instructions for processor 1104. A storage device 1110, such as a magnetic disk or optical disk, is provided and coupled to bus 1102 for storing information and instructions.

Computer system 1100 may be coupled via bus 1102 to a display 1112, such as a cathode ray tube (CRT), for displaying information to a computer user. An input device 1114, including alphanumeric and other keys, is coupled to bus 1102 for communicating information and command selections to processor 1104. Another type of user input device is cursor control 1116, such as a mouse, a trackball, or cursor direction keys for communicating direction information and command selections to processor 1104 and for controlling cursor movement on display 1112. This input device typically has two degrees of freedom in two axes, a first axis (e.g., x) and a second axis (e.g., y), that allows the device to specify positions in a plane.

Computer system 1100 may implement the techniques described herein using customized hard-wired logic, one or more ASICs or FPGAs, firmware and/or program logic which in combination with the computer system causes or programs computer system 1100 to be a special-purpose machine. According to one embodiment, the techniques herein are performed by computer system 1100 in response to processor 1104 executing one or more sequences of one or more instructions contained in main memory 1106. Such instructions may be read into main memory 1106 from another storage medium, such as storage device 1110. Execution of the sequences of instructions contained in main memory 1106 causes processor 1104 to perform the process steps described herein. In alternative embodiments, hard-wired circuitry may be used in place of or in combination with software instructions.

The term “storage media” as used herein refers to any non-transitory media that store data and/or instructions that cause a machine to operate in a specific fashion. Such storage media may comprise non-volatile media and/or volatile media. Non-volatile media includes, for example, optical or magnetic disks, such as storage device 1110. Volatile media includes dynamic memory, such as main memory 1106. Common forms of storage media include, for example, a floppy disk, a flexible disk, hard disk, solid state drive, magnetic tape, or any other magnetic data storage medium, a CD-ROM, any other optical data storage medium, any physical medium with patterns of holes, a RAM, a PROM, and EPROM, a FLASH-EPROM, NVRAM, any other memory chip or cartridge.

Storage media is distinct from but may be used in conjunction with transmission media. Transmission media participates in transferring information between storage media. For example, transmission media includes coaxial cables, copper wire and fiber optics, including the wires that comprise bus 1102. Transmission media can also take the form of acoustic or light waves, such as those generated during radio-wave and infra-red data communications.

Various forms of media may be involved in carrying one or more sequences of one or more instructions to processor 1104 for execution. For example, the instructions may initially be carried on a magnetic disk or solid state drive of a remote computer. The remote computer can load the instructions into its dynamic memory and send the instructions over a telephone line using a modem. A modem local to computer system 1100 can receive the data on the telephone line and use an infra-red transmitter to convert the data to an infra-red signal. An infra-red detector can receive the data carried in the infra-red signal and appropriate circuitry can place the data on bus 1102. Bus 1102 carries the data to main memory 1106, from which processor 1104 retrieves and executes the instructions. The instructions received by main memory 1106 may optionally be stored on storage device 1110 either before or after execution by processor 1104.

Computer system 1100 also includes a communication interface 1118 coupled to bus 1102. Communication interface 1118 provides a two-way data communication coupling to a network link 1120 that is connected to a local network 1122. For example, communication interface 1118 may be an integrated services digital network (ISDN) card, cable modem, satellite modem, or a modem to provide a data communication connection to a corresponding type of telephone line. As another example, communication interface 1118 may be a local area network (LAN) card to provide a data communication connection to a compatible LAN. Wireless links may also be implemented. In any such implementation, communication interface 1118 sends and receives electrical, electromagnetic or optical signals that carry digital data streams representing various types of information.

Network link 1120 typically provides data communication through one or more networks to other data devices. For example, network link 1120 may provide a connection through local network 1122 to a host computer 1124 or to data equipment operated by an Internet Service Provider (ISP) 1126. ISP 1126 in turn provides data communication services through the world wide packet data communication network now commonly referred to as the “Internet” 1128. Local network 1122 and Internet 1128 both use electrical, electromagnetic or optical signals that carry digital data streams. The signals through the various networks and the signals on network link 1120 and through communication interface 1118, which carry the digital data to and from computer system 1100, are example forms of transmission media.

Computer system 1100 can send messages and receive data, including program code, through the network(s), network link 1120 and communication interface 1118. In the Internet example, a server 1130 might transmit a requested code for an application program through Internet 1128, ISP 1126, local network 1122 and communication interface 1118.

The received code may be executed by processor 1104 as it is received, and/or stored in storage device 1110, or other non-volatile storage for later execution.

In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense. The sole and exclusive indicator of the scope of the invention, and what is intended by the applicants to be the scope of the invention, is the literal and equivalent scope of the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. 

What is claimed is:
 1. A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising: identifying a request to return a first value, of a first type, from a first field that (a) corresponds to a particular object and (b) is referenced by a particular field name; determining that the particular object is associated with a particular type; determining that the particular type comprises: (a) a second field that (i) is referenced by the same particular field name indicated by the request and (ii) is of the first type; (b) a third field that (i) is referenced by the same particular field name indicated by the request and (ii) is of a second type different from the first type; executing a conversion function to convert a second value, that is fetched from the third field and is of the second type, into the first value of the first type; returning the first value of the first type.
 2. The medium of claim 1, wherein the second field and the third field are different versions of a migrated field.
 3. The medium of claim 1, wherein the request comprises fetching the second value for the third field corresponding to the particular object.
 4. The medium of claim 1, wherein the request is identified in a class file.
 5. The medium of claim 1, wherein: the request includes a symbolic reference to the first field; the operations further comprise: resolving the symbolic reference as a direct reference to the third field; identifying a memory address of the third field corresponding to the particular object; and fetching the second value from the memory address.
 6. The medium of claim 1, wherein an annotation associated with at least one of the second field and the third field indicates the second field migrated to the third field.
 7. The medium of claim 1, wherein executing the conversion function is responsive to: determining that the second field was migrated to the third field.
 8. The medium of claim 1, wherein executing the conversion function is responsive to: determining that the first type indicated by the request and the first type associated with the second field are same.
 9. The medium of claim 1, wherein determining that the particular type comprises (a) the second field and (b) the third field comprises: determining that a declaration or definition of the particular type includes a first declaration of the second field and a second declaration of the third field.
 10. A method, comprising: identifying a request to return a first value, of a first type, from a first field that (a) corresponds to a particular object and (b) is referenced by a particular field name; determining that the particular object is associated with a particular type; determining that the particular type comprises: (a) a second field that (i) is referenced by the same particular field name indicated by the request and (ii) is of the first type; (b) a third field that (i) is referenced by the same particular field name indicated by the request and (ii) is of a second type different from the first type; executing a conversion function to convert a second value, that is fetched from the third field and is of the second type, into the first value of the first type; returning the first value of the first type; wherein the method is executed by at least one device including a hardware processor.
 11. The method of claim 10, wherein the second field and the third field are different versions of a migrated field.
 12. The method of claim 10, wherein the request is identified in a class file.
 13. The method of claim 10, wherein: the request includes a symbolic reference to the first field; the operations further comprise: resolving the symbolic reference as a direct reference to the third field; identifying a memory address of the third field corresponding to the particular object; and fetching the second value from the memory address.
 14. The method of claim 10, wherein executing the conversion function is responsive to: determining that the second field was migrated to the third field.
 15. The method of claim 10, wherein determining that the particular type comprises (a) the second field and (b) the third field comprises: determining that a declaration or definition of the particular type includes a first declaration of the second field and a second declaration of the third field.
 16. A system, comprising: at least one device including a hardware processor; and the system configured to perform operations comprising: identifying a request to return a first value, of a first type, from a first field that (a) corresponds to a particular object and (b) is referenced by a particular field name; determining that the particular object is associated with a particular type; determining that the particular type comprises: (a) a second field that (i) is referenced by the same particular field name indicated by the request and (ii) is of the first type; (b) a third field that (i) is referenced by the same particular field name indicated by the request and (ii) is of a second type different from the first type; executing a conversion function to convert a second value, that is fetched from the third field and is of the second type, into the first value of the first type; returning the first value of the first type.
 17. The system of claim 16, wherein the second field and the third field are different versions of a migrated field.
 18. The system of claim 16, wherein the request is identified in a class file.
 19. The system of claim 16, wherein: the request includes a symbolic reference to the first field; the operations further comprise: resolving the symbolic reference as a direct reference to the third field; identifying a memory address of the third field corresponding to the particular object; and fetching the second value from the memory address.
 20. The system of claim 16, wherein executing the conversion function is responsive to: determining that the second field was migrated to the third field.
 21. The system of claim 16, wherein determining that the particular type comprises (a) the second field and (b) the third field comprises: determining that a declaration or definition of the particular type includes a first declaration of the second field and a second declaration of the third field.
 22. A non-transitory computer readable medium comprising instructions, which when executed by one or more hardware processors, cause performance of operations comprising: identifying a request to return a first value, of a first type, from a first field that (a) corresponds to a particular object and (b) is referenced by a particular field name; determining that a prior version of the first field, that is referenced by the same particular field name indicated by the request, is of a second type; determining that a current version of the first field, that is referenced by the same particular field name indicated by the request, is of a third type; responsive to determining that (a) the second type and the first type are same and (b) the third type and the first type are different: executing a conversion function to convert a second value, that is fetched from the first field and is of the third type, into the first value of the first type; returning the first value of the first type.
 23. The medium of claim 22, wherein determining that the prior version of the first field is of the second type and determining that the current version of the first field is of the third type comprises: determining that a declaration or definition of the particular type includes a first declaration of the prior version of the first field and a second declaration of the current version of the first field. 